Apply more let/const transforms.

_basePickBy.js

@@ -12,14 +12,13 @@ import castPath from './_castPath.js';
  * @returns {Object} Returns the new object.
  */
 function basePickBy(object, paths, predicate) {
-  var index = -1,
-      length = paths.length,
-      result = {};
+  let index = -1;
+  const length = paths.length;
+  const result = {};
 
   while (++index < length) {
-    var path = paths[index],
-        value = baseGet(object, path);
-
+    const path = paths[index];
+    const value = baseGet(object, path);
     if (predicate(value, path)) {
       baseSet(result, castPath(path, object), value);
     }